Spectroscopic Methods to Measure Low-energy Photoelectron from Solids in Medium Vacuum Using a Classic Retarding Field Analyzer

Understanding the electronic structures of materials under environmental conditions, such as in the atmosphere, has gained significant attention for applications in catalytic chemistry and improving air stability in organic electronics. Recently, near-ambient pressure photoelectron spectroscopy (NAP-PES) has been developed, enabling measurements even at atmospheric pressure using differential pumping and powerful light sources. Although NAP-PES is a well-established and powerful method, it faces practical challenges. First, the environment around the sample surface is dynamic, influenced by gas flow due to differential pumping. Second, the apparatus is expensive, and machine time is limited, particularly in synchrotron radiation experiments. In contrast, photoelectron yield spectroscopy (PYS) for measuring ionization energy and work function, which is simpler and more affordable than NAP-PES, has been used in both vacuum and atmospheric environments in semiconductor device research. However, PYS cannot determine the density of states (DOS). This study focuses on ultraviolet photoelectron spectroscopy (UPS) using a classic retarding field analyzer (RFA) in current-detection mode. By measuring UPS spectra of highly oriented pyrolytic graphite under varying vacuum pressures, ranging from ultrahigh vacuum to over 100 Pa, we found that photoelectron kinetic energy distribution could be obtained even at pressures where the electron flight length in the RFA is seven times the photoelectron’s mean free path. The feasibility of UPS, PYS, and constant-final-state yield spectroscopy was further demonstrated by measuring the spectra of a C60 film under different vacuum conditions. Changes in the DOS of the C60 film due to oxygen and/or water molecules under medium vacuum conditions were clearly observed. Thus, photoelectron spectroscopy using an RFA in low to medium vacuum conditions is feasible. The apparatus is inexpensive and robust, offering potential for future use by researchers in device development to survey various materials for electronics under practical conditions.